A road-rail vehicle is a vehicle which can operate both on rail tracks and a conventional road. They are also called hi-rail, from highway and rail, or variations such as high-rail, HiRail, Hy-rail, etc.

They are often converted road vehicles, keeping their normal wheels with rubber tires, but fitted with additional flangedsteelwheels for running on rails. The rail wheels are raised and lowered as needed. Purpose-built road-rail vehicles also exist.

Road-rail technology is believed to have been developed by Fairmont Railway Motors in the 1940s to improve flexibility of vehicle use. Fairmont's key product, motor section cars, limited the ability for maintenance crews to travel. Fairmont was bought by Harsco in the late 1980s.

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Such vehicles are normally used for railroad right-of-way maintenance during engineering possessions of the line. They can be driven on roads to near the site and then convert to rail vehicle for the final journey to the worksite. This avoids the complex maneuvers that would be associated with a road vehicle accessing the worksite if the worksite is not near a road. Since they are normally converted road vehicles, they would not fare well in a collision with a heavy rolling stock and therefore can normally only drive on rail tracks under an engineering possession. They are generally designed to be insulated, thus they do not activate track (signaling) circuits although some rail operators, normally those operating remote lines without boom gates etc. prefer them to be non-insulated so that they are detectable by train safety systems.

Attempts have been made over the years to design buses and coaches that could operate on both roads and railway tracks. These attempts were never particularly successful. Some were carried out in Britain during the 1930s, on the Nicky Line by LMS, using a Ro-Railer. In Australia, the New South Wales Railways tried road-rail vehicles during the 1970s on New South Wales routes.

The RTL is a truck prime mover with railway wheels that can be lowered when operating on the railway tracks. It was pioneered by in the early 1990s by Australian National and later refined in Victoria. The idea was to have a locomotive that could transfer from one branch line carrying mainly wheat to another parallel branch line, where the rail connection is very roundabout. The RTL suffered a number of disadvantages. Loads were severely limited when the track was steeply graded. The rubber traction wheels slipped on the steel rails when wet. The life of the rubber tires was rather short.

In Belgium, the company UCA produces the UCA-TRAC, rail/road vehicles based on the chassis of a JCB Load-All (UCA-TRAC B) and JCB Fastrac (UCA-TRAC F). The UCA-TRAC provides traction through its rubber wheels.

In Australia, similar vehicles are built by Aries Rail using Volvo loaders and AUSA telehandlers. These vehicles are able to be used for other purposes such as shovelling or forklifting whilst not operating on-rail.

Road-rail vehicle train brake control

Such vehicles often have cabin mounted train brake controls so that the driver can apply and release the train brakes during shunting manoeuvres. In order to charge the train brake air hose, an air compressor needs to be fitted to the road-rail vehicle.

Self-propelled maintenance vehicles for maintenance of the track and for shunting wagons are much more convenient to use if they can transfer to the road to reposition or otherwise get out of the way. Because relatively light loads are involved, the problems plaguing the Road Transferable Locomotive are avoided.

An example would be a forklift truck fitted with railway wheels and a coupling with which to shunt a wagon or two.

In Belgium, the company UCA bvba has been constructing rail/road vehicles since 1981. UCA started with converting WF-trac and MB-trac for rail traction uses. They built rail car movers, shunting locotractors and other rail/road vehicles. Best known is the UCA-TRAC, based on the chassis of a JCB Load-All (UCA-TRAC B) and Fastrac (UCA-TRAC F). The UCA-TRAC provides traction through its rubber wheels.

Heavy technical assistance shunting vehicle truck with a crane and drawbar for streetcars

Road rail vehicles, particularly those used for inspection purposes, have been involved in a number of serious incidents, including deaths. There has been ongoing discussion [clarification needed] regarding maintenance and inspection standards, including load and load distribution, to minimise the risk of failures.

In Australia, the Rail Industry Safety and Standards Board (RISSB) is working with manufacturers and operators to produce an Australian Standard to which road-rail vehicles must comply.

When operating in road-going mode, drivers have to remember that the dynamics of the vehicle will be changed due to the increased weight at the front and rear of the chassis. Some manufacturers have developed systems that allow the rail wheels to be stored almost entirely inside the original bodywork[1] thus moving the centre of mass closer to the road axles. This greatly improves the on-road driving performance of the vehicle.

"Evans Auto-Railer" was a pioneer in the US in the 1930s and 40s.[2] Evans seems to have produced all road/rail vehicle adaptations for the U.S. Military during WWII, but were rapidly overtaken by Fairmont immediately post-war.

"Aries Hyrail" branded vehicles have been manufactured in Australia for several decades. Aries Hyrail vehicles continue to be manufactured by Aries Rail